The invention is based on a fuel injection nozzle as generally defined hereinafter. In injection nozzles of this type, in order to terminate the pre-injection phase a certain volume of the delivered fuel is "swallowed" by the deflecting reservoir piston, so that the valve needle is temporarily braked or even, depending on the design, returned to the valve seat. Furthermore the effective pressure area on the valve needle is reduced in size by the size of the second pressure shoulder, so that to move the valve needle further or to reopen the valve a notably higher fuel pressure is required, and the primary quantity of the fuel is then injected at this higher pressure. The reduced effective pressure area is maintained until the end of the injection event, thereby producing an exact end of injection with a high closing pressure.
In a known injection nozzle of this type (U.S. Pat. No. 2,558,148), the reservoir piston is displaceably disposed in a cylindrical bore in the interior of the valve needle. The mouth of the auxiliary fuel conduit is located at one end of the cylindrical bore, which forms the second pressure shoulder of the valve needle. The reservoir piston has a valve tang extension of offset diameter, which is pressed by the restoring spring against the mouth of the auxiliary conduit and monitors the mouth. At the end of the pre-injection phase the auxiliary conduit is opened in accordance with the fuel pressure, which is extended in a virtually unthrottled manner into the auxiliary conduit and there acts upon the valve face of the valve tang extension on the reservoir piston. As soon as the latter has uncovered the mouth of the auxiliary conduit, the fuel pressure in the cylindrical bore acts upon the entire cross-sectional surface area of the reservoir piston and rapidly translates the latter into its terminal position, in which it is supported on the housing and where it remains until approximately the end of injection. Initiating the primary injection phase in accordance with pressure has the disadvantage that if the opening pressures of the valve needle and deflecting piston are varied, it becomes possible for the deflecting piston to open without a pre-injection having taken place beforehand.